The front-end servers register their fully qualified domain name (FQDN) as A records in DNS. When the Enterprise pool is created, the pool FQDN is registered to return from DNS the list of IP addresses of all the front-end servers. The client attempts to connect to one of the IP addresses that were returned. If this connection fails, the client attempts to connect to the next IP address in the list until the connection succeeds.

Server Failure and Recovery

When a server fails, the physical registrar sequence is updated to show the server as unavailable and shared amongst all surviving servers by using a server-server heartbeat. Users are redirected to the next server in their logical registrar sequence and are then connected in backup mode. The server will be recovered returning the physical registrar sequence back to its original state.

Server Commission and Decommission

When topology changes occur, the logical registrar sequence is recalculated for all users. Some users are re-homed to a different front-end server in the same pool. When the server is fully operational, the heartbeat process updates the physical registrar sequence. This results in the batched re-registration process. Decommission is very similar to server failure, with the exception of the re-home to a new primary registrar being part of the decommission process. The topology change results in the recalculation of the logical registrar sequence. This step doesn't happen in a server failure.

You can use DNS load balancing for the SIP traffic on Front End pools and Director pools. With DNS load balancing deployed, you still need to also use hardware load balancers for these pools, but only for HTTP and Distributed Component Object Model (DCOM) traffic. The hardware load balancer is used for HTTP traffic from clients over ports 443 and 80, and for DCOM traffic over port 135 from administrators performing user moves.

Although you still need hardware load balancers for these pools, their setup and administration will be primarily for HTTP traffic, which the administrators of hardware load balancers are accustomed to.

A new feature called server draining enables you to take a server offline without any loss of service to users. When a server is drained it stops taking new connections and calls. These new connections and calls are routed through other servers in the pool. A server being drained allows its sessions on existing connections to continue until they naturally end. When all existing sessions have ended, the server is ready to be taken offline